Chlorine gas is a highly toxic respiratory irritant that is considered a chemical threat agent because of the possibility that it could be released in industrial accidents or terrorist attacks. Acute effects of chlorine inhalation include dyspnea, hypoxemia, pneumonitis, and pulmonary edema. Longer term consequences of chlorine inhalation include pulmonary function impairment and airway structural changes that have been observed in some exposed individuals. We have developed a mouse chlorine inhalation model in which animals develop pulmonary edema, inflammation, airway obstruction, and airway hyperreactivity within one day after exposure. In new studies related to the current application, we have characterized injury and repair of the lung at longer times after chlorine inhalation. We have observed that airways with a partial loss of epithelium are repaired quickly by the proliferation and differentiation of epithelial cells that survive chlorine exposure. In contrast, portions of airways with few surviving epithelial cells are repaired inefficiently, and fibroproliferative lesions develop at such sites within a week after chlorine exposure. This development of airway fibrosis is associated with impaired lung function, including increased respiratory system resistance and airway hyperreactivity to inhaled methacholine. In the current application, we propose to develop countermeasures that can be administered following a chemical attack or accidental release to prevent chlorine-induced airway disease. The hypothesis to be tested in the proposed studies is that inefficient repair of airway epithelium following chlorine lung injury leads to airway fibrosis and impaired lung function; agents that mimic anti-fibrotic effects of airway epithelium or stimulate airway repair will ameliorate chlorin- induced airway disease. Specific Aim 1 will characterize airway epithelial repair, airway fibrosis, and lung function impairment following chlorine exposure in mice. Specific Aims 2-5 will evaluate the efficacy of the following potential countermeasures for the treatment of chlorine-induced airway disease: the prostanoid receptor agonists butaprost and iloprost; the long-lasting -agonist formoterol and the type 4 phosphodiesterase inhibitor rolipram; stimulation of plasminogen activation with urokinase-type plasminogen activator or the plasminogen activator inhibitor-1 inhibitor tiplaxtinin; and manipulation of Wnt/-catenin signaling with lithium chlorie or pyrvinium. Specific Aim 6 will involve development of a ferret model of chlorine-induced airway disease and testing of countermeasures in this non-rodent species. The proposed studies are expected to identify a countermeasure for prevention of chronic airway disease that develops as a result of an acute high level exposure to chlorine.